There has been great interest recently in the surface chemistry and textile field about so-called super-hydrophobic surfaces and their resulting “self-cleaning” properties. This self-cleaning effect of the super-hydrophobic surface is principally controlled by how a liquid (typically water) interacts with the surface of, for instance, a textile substrate. The interaction between a liquid, air and a flat solid surface is governed by Young's equation and is often described by a contact angle between a liquid droplet and a solid surface. The contact angle is determined by the interfacial tensions between the solid-liquid, solid-air, and liquid-air interfaces. The solid surface may be, for example, hydrophilic (having a strong affinity for or the ability to absorb water) or hydrophobic (lacking affinity for or the ability to absorb water). Typically, a surface having a high contact angle is more hydrophobic that a surface having a lower contact angle. Young's equation and a schematic illustration of the solid/liquid interface contact angle are shown in FIG. 1.
Young's equation is useful to determine the contact angle of a droplet (such as water) on a uniform flat surface. So far, the highest water contact angle on a flat solid surface is around 120 degrees for low surface energy fluorinated materials. To further improve the contact angle, surface structure modifications are needed. It is known that the contact angle can be substantially higher on a rough surface than on a flat surface with the same chemical nature. For highly roughened hydrophobic surfaces with relatively high aspect ratios, which allows a substantial amount of air to be trapped underneath a water droplet, the contact angle can be greater than 150 degrees (i.e., a super-hydrophobic surface). In both cases having roughened surfaces, Young's equation is modified, since the surface area in contact with the water droplet has changed. When a drop of water is put on such a super-hydrophobic surface, the water droplet beads up like a sphere and may roll off when the surface is tilted slightly. It is noted, however, to achieve a low roll off angle, which is defined as the minimum tilt angle at which a drop starts to roll, high contact angle alone is not sufficient. It also requires low contact angle hysteresis, which is defined as the difference between the advancing angle and receding angle. The advancing angle is the contact angle of a liquid that is observed while increasing the volume of a drop that is pinned to a surface by, for instance, a syringe. Decreasing the volume of the very same drop of liquid results in the receding angle. If a liquid drop has a high advancing contact angle, but has a substantially lower receding contact angle (i.e. with a substantial contact angle hysteresis), the drop may bead up but a higher roll off angle is required for the drop to roll. It may furthermore leave a trail of liquid behind it on the surface. On the other hand, if a liquid drop has both high advancing and receding contact angles without substantial hysteresis, the drop is able to bead up and roll off at a small tilt angle without leaving a significant trail of liquid behind it on the surface (i.e., a super liquid repellent surface). This beading and rolling action allows the liquid drop to roll off the surface and carry away loose dirt or dust with the drop, which leads to the “self-cleaning” property. On a smooth surface with lower contact angles, on the other hand, the liquid drop does not pick up the surface dirt. It typically will only redistribute the dirt. These scenarios are schematically illustrated in FIG. 2.
The super water repellent self-cleaning approach is found in nature in a water loving plant called the Lotus. The Lotus plant is a wetland species native to Asia, and it is revered as a symbol of purity because it maintains its cleanliness, although it grows out of muddy water. The Lotus plant's super water repellent self-cleaning properties come from a combination of rough physical surface structures and hydrophobic surface chemistries. Specifically, the surface of the Lotus leaf exhibits two levels of surface irregularity, as shown in FIG. 3. Each leaf surface (see also FIGS. 3A and 3B) is covered in an array of tiny bumps, about 5-10 μm high, and about 10-15 μm apart. A scale showing 20 microns is evident in all three of these Figures, enabling an estimation of the approximate size of the surface features. This set of uneven surface structures is also covered with much smaller waxy, hydrophobic crystals, measuring about 1 nm or less in diameter. It is believed that this “dual” level of structure on the Lotus leaf surface increases the overall surface roughness, which correspondingly boosts its effective hydrophobicity. The two-tier structure, with hydrophobic “valleys,” reduces the likelihood of surface air spaces being invaded by water, for instance, from condensation, evaporation, or high impact rainstorms, since it is not energetically favorable for the water to fill the valley. Thus, it is believed that the physical surface structure of the Lotus leaf, coupled with its surface chemistry (hydrophobic wax crystals), allows the surface to act as a super water repellent, self-cleaning surface where water beads up and rolls off, carrying away loose dirt from the surface. This super water repellent self-cleaning action in the presence of water on surfaces is often known as the “Lotus-Effect”.
In analogy to the Lotus plant, textile surfaces that repel fluids and demonstrate super liquid repellency and associated self-cleaning properties would be able to maintain their appearances more effectively. Thus, a textile surface having properties similar to the Lotus plant may be highly desirable. A textile substrate having Lotus properties might be used to make highly water repellent and/or fluid stain repellent fabrics for applications, such as, for example, raingear, boat covers, awnings, lawn furniture, apparel, etc. In providing fabrics that are water and/or fluid stain repellent, the fabrics may also be resistant to bacterial and mold growth, since water and/or other liquids would not tend to collect or wick into such surfaces to provide a breeding ground for the bacteria and mold.
Repellent textile substrates have been available in the market for some time. For example, fluorocarbon treatments are known to provide a repellent finish to such substrates. Many fluorocarbon-containing compositions are known to provide repellency to both water and oil when applied to a textile substrate. However, to obtain the high contact angles and low roll off angles required for a super water repellent surface, a surface structure similar to that of the Lotus leaf is typically required. There have been a variety of efforts by others to produce such Lotus-like surface structures, such as in U.S. Pat. No. 6,068,911 to Shouji et al.; US Patent Application Publication No. 2002/0016433 to Keller et al.; and US Patent Application Publication Nos. 2002/0150723, 2002/0150724, 2002/0150725, 2002/0150726, 2003/0013795, and 2003/0147932 to Creavis Gesellschaft Fuer Techn. Und Innovation MBH. These references disclose means to use particles to build rough structures on primarily smooth flat surfaces. Textile substrates treated according to these references generally do not provide durable repellency because they often lose most or all of their repellency when laundered or when abraded during normal use. Thus, textile substrates that retain advantageous superior liquid repellent properties, after laundering or exposure to abrasion from normal use, would be very desirable and novel.
U.S. Pat. No. 5,968,642 to Saito is directed to an article having a water-repellent fluororesin surface. The article has a water-repellent fluororesin surface composed of an irregularly porous material which is formed by irregularly stacking fluororesin particles, with an average diameter of no more than 40 microns, over one another. Again, this patent is directed to substrates that have a flat surface, such as an aluminum sheet. Furthermore, it fails to demonstrate its applicability to complex structured textile substrates having irregular surfaces, and it fails to demonstrate durability of the treatment against laundering and abrasion for textile applications.
U.S. Pat. No. 6,649,266 to Gross et al. is directed to substrates such as glass or metal with a microstructured surface for easy-to-clean systems and methods to produce such substrates. The treatment is comprised of a composition that includes condensates of one or more hydrolysable compounds. At least some of these compounds contain both hydrolysable and non-hydrolysable groups with a ratio from 10:1 to 1:2. Inorganic nanoparticles are used to produce the microstructured surface before the coating composition is applied to the substrate. The microstructured surface may also be obtained by embossing the coating composition, before or during drying and/or curing, with an embossing die. The contact angle, with respect to water or hexadecane on such microstructured substrates, is at least 5 degrees higher than the contact angle of a corresponding smooth surface. While the methods taught in this patent may work well on flat surfaces, the teachings therein fail to demonstrate its applicability to complex-structured textile substrates and further, fail to demonstrate durability of the treatment against laundering and abrasion for typical textile applications.
US Patent Application No. 2003/0096083 to Morgan et al. relates to surfaces of objects, in particular containers for receiving liquid, comprising a surface which is extremely hydrophobic and to a method for producing such surface. According to the reference, a surface structure is created by fine blasting the surface with suitable blasting material and/or embossing with an appropriate embossing step or etching by means of a suitable etching material. This reference fails to teach how to treat complex structured textile substrates and fails to demonstrate durability of the treatment against laundering and abrasion for typical textile applications.
One publication, WO 01/75216, discloses a method for applying a finishing layer to a textile carrier material. According to the publication, a water repellent layer or an oil repellent layer is applied to a carrier material of a group of fibers, tissues and fabrics. The water or oil repellent finishing layer comprises at least two water or oil repellent components. A first component comprises at least one dispersing agent and a second component comprises at least one dispersed phase or a colloid. The dispersing agent and the dispersed phase are present in a gel state. The colloids of the dispersed phase are distributed in the dispersing agent in an anisotropic manner in such a way that the colloids are present in concentrated form in the region of the finishing surface, which forms a phase boundary layer between the finishing layer and the surrounding atmosphere. An essential feature of this approach is the use of a dispersion system as a “guest-host” system which allows spatial self organization of the finishing components. The “guest” component becomes phase separated from the “host” component and concentrated on top of the finishing layer with a columnar structured micro rough surface. This approach relies on the chemical mixtures to be inherently phase instable. Thus, the process is difficult to control and may cause problems during large-scale manufacturing.
Another publication, WO 02/084016, is directed to a flat textile structure with self-cleaning and water-repellent surfaces that are composed of (a) at least one synthetic and/or natural textile base material A and (b) one artificial and, at least partially, hydrophobic surface with elevations and depressions from particles that are firmly linked with the base material A without glues, resins or lacquers. The flat textile structures are obtained by treating the base material A with at least one solvent that contains the particles in an undissolved state and then removing the solvent so that at least a part of the particles are firmly linked with the surface of the base material A. However, this publication fails to demonstrate low dynamic rolling angles and durability of the treatment against laundering and abrasion.
Yet another publication, WO 02/084013, is directed to a polymer fiber which has a self-cleaning and water-repellent surface and which is comprised of: (a) at least one synthetic fiber material A and (b) a synthetic and, at least partially, hydrophobic surface with elevations and depressions made of particles that are joined to the fiber material A in a fixed manner without the use of adhesives, resins or varnishes. The polymer fiber is obtained by treating the fiber material A with at least one solvent that contains the particles in undissolved form and then removing the solvent so that at least a portion of the particles are joined to the surface of the synthetic fiber material A in a fixed manner. Again, this publication fails to demonstrate low dynamic rolling angles and durability of the treatment against laundering and abrasion.
The prior art systems that use hydrophobic particles and binders in solvent systems on film, metallic, or ceramic surfaces to achieve super-hydrophobic self-cleaning properties have significant drawbacks, such as poor durability, when applied to textile substrates. The coatings applied to the surfaces wear off easily during normal use. FIGS. 4 and 4A show a fiber from a woven textile substrate that have been treated with hydrophobic particles according to procedures described in US Patent Application No. 2002/0016433 A1 to Keller et al. These textile substrates may achieve some level of repellency, but the repellency is not durable against laundering of the fibers or the textile substrate. The hydrophobic particles are easily and undesirably removed from the surface.
In summary, most prior art systems that attempt to achieve the Lotus-Effect, as described herein, are directed to substrates that have a flat smooth surface, such as glass, ceramic, metal sheet, plastic film, etc. On such smooth surfaces, the rough structures required for super-hydrophobic self-cleaning properties may be achieved by using particles alone. On a typical textile substrate, such as a woven fabric, a complex surface topology already exists. For instance, millimeter scale structures are created by the weaving of yarns; 10 to 100 micrometer scale structures are created by fibers within the yarn. Furthermore, the textile substrates are mechanically flexible. On such complex structured flexible textile substrates, particles alone typically are not sufficient to build desired rough structures which exhibit the Lotus-Effect and that are durable against laundering and abrasion for textile applications.
Surface structures can be imparted to a textile substrate by other ways. For instance, mechanical means of treating surfaces of textiles are known. Desirable hand (or feel) or other properties may be provided to fabrics using treatments that involve techniques of abrasion, sanding, or napping the fabric. However, the usual goal of mechanically treating textile surfaces is to break fibers and produce broken-fiber “hairs” that make a surface feel softer. Therefore, the purpose of such conventional mechanical treatment is typically to modify the hand of a substrate rather than to roughen the surfaces of fibers without breaking the fibers. For example, U.S. Pat. Nos. 6,112,381; 5,815,896; 4,512,065; 4,316,928; and 4,468,844 describe various types of mechanical treatments for the surfaces of textile substrates. However, textile substrates that are mechanically treated by these types of processes are found to lack the desired roughness structures that are needed for the lotus-effect properties.
Thus, the need exists for a composition and/or a method of treating or coating textile substrates that results in a textile substrate having a super liquid repellent self-cleaning surface. A surface structure and method for treating a textile substrate that will afford surface properties exhibiting a high degree of repellency, as measured by low dynamic rolling angle (DRA), is very desirable. A surface structure and method for treating a textile substrate that will afford surface properties exhibiting a high degree of repellency, that is durable after multiple launderings and/or abrasions, is also desirable.
Surprisingly, we found that, on such complex structured textile substrates, superior liquid (e.g. water and oil) repellent properties can be obtained by mechanically roughening only a portion, for example 10% or more, of the fibrous textile substrate surface without substantially breaking fibers. Durability of the repellent properties may be enhanced by following mechanical treatment of the substrate with a chemical treatment using, for example, fluorocarbon-containing repellent compositions. Particles may be used in combination with the repellent chemical treatment. The mechanical roughening treatment of the present invention occurs in a treatment regime very different from that used in the prior art. The resulting treated textile substrates are believed to be durable against multiple cycles of laundering and/or abrasion experienced during normal use because the mechanically roughened structures on the surfaces of the fibers are part of the fibers themselves and are attached durably and directly to the fibers.